A suburban woman has told a US judge in Philadelphia that she had worked feverishly online under the name Jihad Jane to support Muslim fighters and moved overseas to further her plan to kill a Swedish artist who had offended Muslims.

Colleen LaRose, 47, faces the possibility of life in prison after pleading guilty on Tuesday to four federal charges, including conspiracy to murder a foreign target, conspiracy to support terrorists and lying to the FBI.

Speaking clearly but quietly, the LaRose told a judge she had never been treated for any mental-health problems and was entering her plea freely. She whispered a few comments to her lawyers, some of them prompting a smile from public defender Mark Wilson.

Wilson declined to comment afterward.

"We'll have a lot to say at sentencing," he said.

LaRose, who was the live-in caretaker for her boyfriend's elderly father, also was building a shadow life online from 2008 to 2009.

According to prosecutors, LaRose "worked obsessively on her computer to communicate with, recruit and incite other jihadists", using screen names including Jihad Jane, SisterOfTerror and ExtremeSister4Life.

LaRose returned to the US in November 2009 and was immediately taken into FBI custody at Philadelphia International Airport.

She remained in secret custody until March, when her indictment was unsealed hours after Irish authorities swept up an alleged "terrorist cell" that included another American women, Jamie Paulin-Ramirez, 32, of Colorado, and her Algerian husband.

LaRose had previously denied the allegations against her and had pleaded not guilty before changing her plea on Tuesday.

But prosecutors said LaRose and her co-conspirators had hoped her all-American appearance and US citizenship would help her blend in while carrying out their plans.

"Today's guilty plea, by a woman from suburban America who plotted with others to commit murder overseas and to provide material support to terrorists, underscores the evolving nature of the threat we face," David Kris, assistant US attorney general, said.

LaRose and Paulin-Ramirez are the rare US women charged with terrorism. Paulin-Ramirez has pleaded not guilty and her lawyer, Jeremy Ibrahim, declined to say whether she will enter a plea or head to trial on May 2.